Range finders based on laser techniques have been successfully developed as disclosed in U.S. Pat. Nos. 5,359,404, 5,612,779, and 5,574,552 granted to Laser Tech Co. A range finder employs a laser diode as its light source can range 1 Km distance with high precision within the range of 1 m. There are three major drawbacks in the techniques employed in the prior arts. First, a method of fast charging and slow discharging is used to achieve the function of expanding the receiving time. Therefore, the time period when the receiving circuit receives the respective echo light after the laser diode transmits the light is expanded and transformed into the distance by counting the time samples at a low speed clock. The desired circuit in the finder costs highly due to the requirement of fast charging and slow discharging. Second, automatical threshold adjustment is used to optimize the sensitivity of the receiving circuit. However, the feedback sensitivity is too high to control. Third, the environmental condition is not considered for the prior art laser light such that the shift phenomenon due to thermal effect may influence the accuracy and stability.